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Condensed-Phase Effects on the Structural Properties of FCHCN– BF and ClCHCN–BF: A Matrix-Isolation and Computational Study 3

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Amanda R Buchberger, Samuel J. Danforth, Kaitlin M. Bloomgren, John A. Rohde, Elizabeth L. Smith, Colin C. A. Gardener, and James Allan Phillips J. Phys. Chem. B, Just Accepted Manuscript • DOI: 10.1021/jp405368a • Publication Date (Web): 29 Aug 2013 Downloaded from http://pubs.acs.org on August 30, 2013

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The Journal of Physical Chemistry

Condensed-Phase Effects on the Structural Properties of FCH2CN–BF3 and ClCH2CN–BF3: A Matrix-Isolation and Computational Study

Amanda R. Buchberger, Samuel J. Danforth, Kaitlin M. Bloomgren, John A. Rohde, Elizabeth L. Smith, Colin C. A. Gardener, and James A. Phillips*

Department of Chemistry, University of Wisconsin–Eau Claire, 105 Garfield Avenue, Eau Claire, WI, 54702

KEYWORDS: Medium Effects, Solvent Effects, Donor-Acceptor Complexes, Molecular Complexes, Gas-Solid Structure Differences

CORRESPONDING AUTHOR: Professor James A Phillips Department of Chemistry, University of Wisconsin–Eau Claire 105 Garfield Avenue, Eau Claire, WI, 54702. 715-836-5399 [email protected]

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Abstract We have measured several IR bands of FCH2CN–BF3 and ClCH2CN–BF3 in solid nitrogen, argon, and neon. These bands include the B-F asymmetric stretch (νBFa), the B-F symmetric stretch (νBFs), the BF3 symmetric deformation or “umbrella” mode (δBFs), and the CN stretch (νCN). For both complexes, the frequencies of these modes for shift across the various media, particularly the B-F asymmetric stretching band, and thus they indicate that the inert gas matrix environments significantly alter the structural properties of FCH2CN–BF3 and ClCH2CN–BF3. Furthermore, the frequencies shift in a manner that parallels the dielectric constant of these media, which suggests a progressive contraction of the B-N distances in these systems, and also that it parallels the ability of the medium to stabilize the increase in polarity that accompanies the bond contraction. We have also mapped the B-N distance potentials for FCH2CN–BF3 and ClCH2CN–BF3 using several density functional and post-Hartree Fock methods, all of which reveal a flat, shelf-like region that extends from the gas-phase minimum (near 2.4 Å) toward the inner wall (to about 1.7 Å). Furthermore, we were able to rationalize the medium effects on the structure by constructing hybrid bond potentials composed of the electrostatic component of the solvation free energy and the gas-phase electronic energy. These curves indicate that the solvation energies are greatest at short B-N distances (at which the complex is more polar), and ultimately, the potential minima shift inwards as the dielectric constant of the medium increases.


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Introduction Chemists have often made a clear distinction between bonding and non-bonding forces, in part because they typically manifest different types of properties, chemical or physical, respectively.1 One interesting facet of molecular complexes, however, is that donor-acceptor bonds can blur this otherwise clear distinction between bonding and nonbonding, with examples that range from weak, intermolecular interactions to bona fide chemical bonds.2,3 The main distinguishing feature between a dative bond and an ordinary polar covalent bond is that the donor and acceptor have the potential to form another bond, but since they are generally stable in their own right, bond formation is not a necessity.2,4 In any event, our interest in these systems stems from the fact that some classes of donor-acceptor complexes, often those that are intermediate in terms of the interaction strength,2,3 are apt to undergo large structural changes in response to changes in their surrounding environment. In such cases, the donor-acceptor bond can contract significantly in transition from the gas phase to condensed-phase media,2,3,5-15 because the medium stabilizes these systems in a way that furthers the extent of bonding in them.2,3,13 The CH3CN–BF3 complex is an extensively studied5,8,9,12-20 example for which medium-induced structural changes have been examined in environments ranging from the pure crystalline solid14,15 to noble gas matrices.8,12,20 Initially, large differences were noted between the gas-phase5 and solid-state14 structures. Specifically, the gas-phase structure has a B-N bond length of 2.01 Å, but this contracts by nearly 0.4 Å upon crystallization to a value of 1.63 Å.14 Another even more extreme case is HCN–BF3, for which the B-N distance contracts by over 0.8 Å in transition from the gas-phase (2.47 Å) to the solid-state (1.64 Å).6,7


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These large gas-solid structure differences observed for CH3CN–BF3 led us to investigate how bulk, condensed-phase environments may affect its structural properties. As such, much of our experimental work has dealt with noble gas matrices,21 and IR spectroscopy has been our primary characterization tool. Indeed, there is a vast body of literature dealing with matrix-IR spectra of donor-acceptor complexes,22 but CH3CN–BF3 and its analogs are particularly interesting because the matrix media alter the structural properties of these systems to a measurable extent.8-10,12,20 Specifically, the measured frequencies of CH3CN–BF3, shift systematically with the dielectric constant of the matrix host in a manner that suggests a progressive contraction of the B-N bond across these media.8,12 Moreover, computations9,13 provide further support for this idea, and provide mechanistic insight. In particular, it seems that an unusually flat donor-acceptor bond potential is the critical underlying feature of this phenomenon.9,13 The general idea is that if the inner, bonded region lies within a few kcal/mol of the outer, nonbonding region of the curve, interactions with a medium (which we have successfully modeled with continuum solvation models, such as PCM23) can alter the potential in a way that shifts the structure towards shorter donor-acceptor bond distances.9,13

A consideration of analogous systems has revealed the impact of substituent effects. For example, the nitrile complexes of BH3 are much stronger than those with BF3 and have much shorter B-N bonds, even in the gas-phase.24 Therefore, condensed-phase effects are predicted to be essentially non-existent because the medium usually acts to compress the B-N bond.2,3,13 Variations in base strength via changes in the nitrile substituent also manifest systematic changes in medium sensitivity. For example, we have seen less pronounced effects with stronger Lewis


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bases, such as C6H5CN and (CH3)3CCN.10 The increased basicity strengthens the complex and the B-N bond is initially shorter (than CH3CN–BF3) the gas phase and thus the condensed-phase effects are less dramatic.10 On the other hand, electron-withdrawing halogen substituents,11 which mute the basicity of the nitrile moiety, lead to more substantial effects. At this point, however, these effects have been demonstrated exclusively via by predicted gas-solid structural differences. Specifically, for FCH2CN–BF3, the B-N bond distance in the gas-phase (via B3PW91/aug-cc-pVTZ) is 2.42 Å, but it contracts by over 0.7 Å to 1.64 Å in the crystal.11 Similarly, the bond in ClCH2CN–BF3 compresses from 2.37 Å in the gas-phase (B3PW91/augcc-pVTZ) to 1.65 Å in the solid-state.11 These predicted gas-solid structure differences are extreme, and rival those observed for HCN–BF3.6,7

In this manuscript, we report additional measurements and computational results that build upon the occurrence of large gas-solid structural differences in the singly halogenated analogs of CH3CN–BF3 (XCH2CN–BF3: X=F, Cl),11 and address the issue of how bulk, inert, condensed-phase media affect the structural properties of these systems. We report the observation of key, structurally-sensitive IR bands for both FCH2CN–BF3 and ClCH2CN–BF3 in several matrix media, including solid neon, nitrogen, and argon. The shifts of these bands across these media provide experimental insight into the degree that the matrix environments affect the structures of these complexes. Furthermore, we have further illustrated these effects with an extensive computational study of the B-N bond potentials, both in the gas-phase and dielectric media.


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Experimental ClCH2CN (99%) and FCH2CN (98%) were obtained from Sigma-Aldrich and subjected to several freeze-pump-thaw cycles before use. BF3 (≥99.5 %) was also acquired from Sigma-Aldrich. All other gases, including argon (99.9995 %), neon (99.9999 %), and nitrogen (99.9999 %), were obtained from Praxair. All gases were used as obtained, without additional purification.

Gas mixtures were prepared in 2 L glass bulbs, using a single bank vacuum manifold evacuated with an oil diffusion pump (all obtained from CHEMGLASS). In some instances, we prepared two separate mixtures, one containing BF3 and host gas and the other containing nitrile, both containing guest-to-host ratios ranging from 1:400 to 1:2400. These mixtures were then mixed via a concentric, dual-deposition plumbing assembly while the sample was being deposited.8,12 In others, both nitrile and BF3 were added to the same bulb with the host gas, with guest-to-host ratios ranging from 1:400 to 1:2400. We varied the relative concentrations of each host gas to a significant extent, in some cases adding up to a four-fold excess of nitrile to facilitate the observation of complex peaks. Optimum signals were obtained with sample bulb containing both guest species, and with guest/host ratios that varied from host-to-host (see below).

The majority of experiments were performed on a previously described12 matrixisolation system based on a Janis SHI-4-5 optical cryostat. Sample temperatures were measured and controlled using a Lakeshore 331 temperature controller with two Si


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diodes, one directly behind the sample window and the other at the end of the second refrigeration stage. The system was evacuated using a corrosive-compatible turbo molecular pump fitted with an inert gas purge to protect pump bearings, and was backed by a corrosive-compatible vacuum pump with a fluorocarbon-based oil.

Most spectra were recorded with a Thermo Nicolet Nexus 670 FTIR spectrometer with resolutions ranging from 1-4 cm-1, and 200-800 scans were averaged per spectrum. The IR beam was routed externally from the spectrometer into the cryostat where it was reflected off a gold-plated sample mirror oriented at a 45-degree angle, which was ultimately routed through some focusing optics and into a DTGS detector. Preliminary experiments (in argon) were performed using another previously described system based on a Cryomech ST15 optical cryostat.8 These spectra were recorded with a Nicolet Avatar 360 FTIR spectrometer with a resolution of 2 cm-1 and 200-800 scans were averaged per spectrum.

Sample deposition rates ranged between 1 to 25 mmol/hour and were controlled using Phillips Granville 203 variable leak valves. Typically, three to five depositions were performed over 3-24 hours, and spectra were collected after each one. Neon matrices were deposited between 5 and 7 K and annealed between 6 and 9 K to facilitate complex formation and aid in the identification of product peaks. Nitrogen matrices were deposited between 12 and 18 K and were annealed between 20 and 30 K. Argon matrices were deposited between 12 and 16 K and were annealed between 25 and 38 K.


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Computational Methods All calculations were performed using Gaussian03 version E0125 or Gaussian09 version A1.26 The starting point for our computational studies was to reproduce the previously-published minimum energy structures,11 which have Cs symmetry and nearly eclipsed conformations. All optimizations were performed with the OPT=TIGHT option, which sets the maximum and root-mean-square (RMS) forces to 1.5 x 10-5 and 1.0 x 10-5 hatrees/bohr, respectively, and the maximum and RMS displacements to 6.0 x 10-5 and 4.0 x 10-5 bohr, respectively. An ultrafine grid was used for every calculation. B-N bond potentials were mapped in a point-wise manner for distances ranging from 1.5 Å to 3.5 Å, in 0.1 Å intervals. All other degrees of freedom aside from the B-N bond distance were optimized at each point. We used several density functional methods,27 including B3PW91, B3LYP, mPW1PW91, M06,28 B98, and B97-2, as well as a handful of postHartree-Fock methods,27 including MP2, MP3, and CCSD. The aug-cc-pVTZ27 basis set was used for all DFT calculations, while 6-311++G(2df,2dp)27 was utilized for all postHF calculations. The exception was that MP2 calculations were performed with both of these basis sets to assess the effect of a smaller basis set for the post-HF calculations. Frequencies and isotopic shifts were calculated at each point on the B-N potentials using B3PW91/aug-cc-pVTZ to provide a reference for the experimental frequencies.

To assess the effect of bulk, dielectric media on the B-N potential, we computed the electrostatic component of the Gibbs energy of solvation using the Polarized


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Continuum Model (PCM)23 for dielectric media with ε values ranging from 1.0 to 40.0. We chose the M06 method for these calculations because the shape of the gas-phase M06 potential resembled the MP2 curve most closely, and the calculated dipole moments at a few points along the curve agreed best with MP2 as well.13 Two types of hybrid energy curves were constructed, one based solely on M06 energies in the gas-phase and dielectric media and one for which the PCM/M06 free energies for the dielectric were added to the CCSD gas-phase potential. Both curves illustrated a similar effect, though the global minima did differ by a few kcal/mol (see below).

Results and Discussion IR Spectra In general, the basis for the assignments of the matrix-IR bands of FCH2CN–BF3 and ClCH2CN–BF3 is as follows. First of all, we closely examined the each spectrum in the regions where we expected the strongest complex bands, the BF asymmetric stretching region (1200 - 1450 cm-1), the BF3 symmetric deformation or “umbrella” mode and BF3 symmetric stretching region (550 - 900 cm-1), and the CN-stretching region (2300 - 2400 cm-1). The quoted ranges match those in the figures below. Initially, we identified reproducible product bands (those that require the presence of both nitrile and BF3) in each region, and monitored the intensity as the samples were annealed, as a slight increase is expected for complex peaks. We then made at least one firm assignment based on a 11B/10B isotopic shift, and the characteristic 4:1 intensity ratio that arises from the relative natural abundance of these isotopes. Subsequently, we linked additional product bands to those of the 1:1 XCH2CN–BF3 complexes by observing consistent relative


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intensities (peak area ratios) as we modulated sample conditions. We note that these peaks were generally weaker than those we have measured previously for analogous complexes, and as such, we fine-tuned the sample conditions to a greater extent than in our previous work on related systems.8,10,12 Ultimately, we assigned for 3 or 4 bands in each host, nitrogen, argon, and neon, and all assigned complex peaks require both nitrile and BF3, grow slightly upon annealing, and exhibit consistent relative intensities across all sample conditions. Frequencies of these bands are listed in Table 1, and representative spectra are shown in Figures 1-6.

Figure 1 shows spectra of FCH2CN and/or BF3 in solid N2 for: the C-N stretching (νCN) region (a), the BF3 asymmetric stretching (νBFa) region (b), and the BF3 symmetric stretching (νBFs) and deformation (δBFs) regions (c). Initially, we identified two product bands at 1299 cm-1 and 1261 cm-1, which exhibited a characteristic 4:1, 11B/10B intensity ratio, and a splitting (38 cm-1) that was reasonably consistent with that for the νBFa band in un-complexed BF3 (51 cm-1), as well as that calculated (53 cm-1, via B3PW91/aug-ccpVTZ) for the analogous νBFa band in gas-phase FCH2CN–BF3. We note that these difference are somewhere larger than those we have observed our previous work,8,10,12 but the calculated shift is somewhat smaller (46 cm-1) for an FCH2CN–BF3 structure with the B-N bond distance constrained to 1.7 Å. As such, we assign the peaks at 1299 cm-1 and 1261 cm-1 to the νBFa bands of FCH2CN–10BF3 and FCH2CN–11BF3, respectively. In the umbrella region, we observed a strong product band as a doublet at 596 and 599 cm-1, as well as several weaker features at 607, 611, 619, and 637 cm-1. The doublet (~598 cm-


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1

) exhibited consistent relative intensities with the νBFa bands, and thus it was a

reasonable candidate for the δBFs band the 11B form of the complex. While the peaks at 637, 611, and 607 cm-1 were all possible candidates for the 10B counterpart, we favor the 619 cm-1 peak because we predict a 20 cm-1 isotope shift (via B3PW91/aug-cc-pVTZ). Thus, we assigned the peaks at 598 cm-1 and 619 cm-1 to the δBFs bands in the FCH2CN11

BF3 and FCH2CN-10BF3, respectively. We observed another weak product band at 833

cm-1, near the region of the BF3 symmetric stretching (νBFs) mode. This band is forbidden in free BF3 but is predicted to become relatively strong upon pyramidal distortion of the BF3 unit. Also, only a small (~1 cm-1) isotope shift is predicted for the gas-phase complex, And though it does increase somewhat as the pyramidal distortion of the BF3 subunit increases, we do not observe an isotopic splitting. On the basis of consistent intensity ratios, we assign the peak at 833 cm-1 to the νBFs band in FCH2CN–BF3 (i.e., not a single B isotopomer). Finally, we observed one additional product band in the C-N stretching region at 2355 cm-1 (which is somewhat obscured by background and impurity CO2). We assign this peak to the C-N stretching mode of FCH2CN–BF3 on the basis of its consistent intensity ratios with the other bands.


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b)

c) a

Figure 1: Representative nitrogen matrix spectra of the: (a) νCN, (b) νBF , s s and (c) νBF and δBF regions containing FCH2CN (i), BF3 (ii), and FCH2CN with BF3 (iii) in a 1:1:800 concentration ratio. Bands assigned to the 1:1 FCH2CN–BF3 complex are noted with asterisks.


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Figure 2 displays representative spectra of FCH2CN and/or BF3 in solid argon in the: νCN (a), νBFa (b), and νBFs and δBFs (c) regions. As with N2, initially identified two product bands in the νBFa region at 1294 and 1334 cm-1 that exhibited a 4:1 intensity ratio, and a splitting (40 cm-1) that was reasonably consistent with that of free BF3 (53 cm-1) and the calculated (B3PW91/aug-cc-pVTZ) shift for the gas-phase FCH2CN–BF3 (52 cm1

). As such, we assigned the peaks at 1294 and 1334 cm-1 to the νBFa bands in FCH2CN–

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BF3 and FCH2CN–10BF3, respectively. In the νBFs /δBFs region, we observed product

peaks at 593, 599, 603, 608, 836, and 839 cm-1. For the former group of these bands, we presume the broad, multiplet structure arises from matrix site splittings, and refer to it via an approximate band canter of 600 cm-1. Unfortunately, a 10B isotopic was not discernable, the intensity ratio with the νBFa peaks is consistent, and as such, we assign the 600 cm-1 peak to the δBFs band of FCH2CN–11BF3. The doublet feature at 839 and 836 cm-1 is presumably the result of a matrix site splitting (it does not exhibit a 4:1 ratio, though a small isotopic shift is predicted). Thus, we will be refer to this band via its approximate center at 837 cm-1, and on the basis of intensity ratios, we assign it to the νBFs mode of FCH2CN–BF3. We also observed a lone product band in the CN stretching region at 2352 cm-1, and again, on the basis of its intensity ratios with other assigned bands, we assign this peak to the νCN of FCH2CN–BF3.


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b)

c) a

Figure 2: Representative argon matrix spectra of the: (a) νCN, (b) νBF , s s and (c) νBF and δBF regions containing FCH2CN (i), BF3 (ii), and FCH2CN with BF3 (iii) in a 2:1:1200 concentration ratio. Bands assigned to the 1:1 FCH2CN–BF3 complex are noted with asterisks.


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Figure 3 shows representative spectra of FCH2CN and/or BF3 in solid neon, for the: νCN (a), νBFa (b), and νBFs and δBFs (c) regions. In this instance, we first turned to the BF3 umbrella and symmetric stretching region, where we observed product bands at 598, 603, and 618 cm-1. The splitting in the 598 cm-1 and 603 cm-1 peaks was attributed to distinct trapping sites, thus we will refer to this band by its approximate center at 600 cm-1. This peak and its apparent counterpart at 618 cm-1 exhibit a consistent 4:1 intensity ratio and a splitting of 18 cm-1. This is comparable to that observed free BF3 (28 cm-1) and the calculated (B3PW91/aug-cc-pVTZ) shift for the gas-phase complex ( 20 cm-1). Thus, we assign the peaks at 600 and 618 cm-1 the δBFs bands of FCH2CN– 11

BF3 and FCH2CN–10BF3, respectively. We also observed an additional product band at

846 cm-1, in the symmetric stretching region, and this peak exhibits consistent intensity ratios with the δBFs bands and we assign it to the νBFs mode of FCH2CN–BF3. In the BF3 asymmetric stretching region, we only observe a single product band centered at 1410 cm-1 (which occurs as a doublet at 1408 and 1413 cm-1 in higher resolution spectra). Unfortunately, we do not observe an isotopic counterpart for this band, but we would expect the band for FCH2CN–10BF3 the BF3 asymmetric near 1450 cm-1, which is obscured due to the strong absorptions of un-complexed BF3. Nonetheless, the 1401 cm-1 band does exhibits consistent intensity ratios, and in spite of the absence of an isotopic counterpart, we assign the 1410 cm-1 peak to the νBFa band of FCH2CN–11BF3. Finally, in the C-N stretching region we observe a product band at 2339 cm-1(though only under fairly rich sample conditions). It did, however, exhibit consistent intensity ratios with all other assigned bands, but due to a lack of reproducibility, we offer only a tentative assignment for the C-N stretching mode of FCH2CN–BF3 at 2339 cm-1.


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a)

b)

c) a

Figure 3: Representative neon matrix spectra of the: (a) νCN, (b) νBF , s s and (c) νBF and δBF regions containing FCH2CN (i), BF3 (ii), and FCH2CN with BF3 (iii) in a 1:1:800 concentration ratio. Bands assigned to the 1:1 FCH2CN–BF3 complex are noted with asterisks.


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Figure 4 shows representative spectra of ClCH2CN and/or BF3 in solid nitrogen for the: νCN (a), νBFa (b), and νBFs and δBFs (c) regions of the spectrum. In the νBFa region we observed a pair of product bands at 1253 and 1291 cm-1 with a 4:1 intensity ratio, and a splitting of 38 cm-1, which is reasonably consistent with the shift for un-complexed BF3 (51 cm-1) and predicted (B3PW91/aug-cc-pVTZ) gas-phase splitting (52 cm-1), and it matches that observed for the corresponding band in FCH2CN–BF3. Therefore, we assign these peaks at 1253 cm-1 and 1291 cm-1 to the νBFa of ClCH2CN–11BF3 and ClCH2CN– 10

BF3, respectively. We also observed product bands at 599, 617, and 836 cm-1 in the

νBFs/δBFs region. The 599 and 617 cm-1 peaks exhibit a 4:1 ratio and a splitting of 17 cm-1, which is relatively consistent with the predicted (B3PW91/aug-cc-pVTZ) splitting of 21 cm-1 and that observed for free BF3 (28 cm-1). Thus, we assigned the 599 and 617 cm-1 peaks to the δBFs modes of ClCH2CN–11BF3 and ClCH2CN–10BF3, respectively. Also, the peak at 836 cm-1 lies in the region of the νBFs band and exhibits consistent intensity ratios with the other assigned bands. Likes its FCH2CN counterpart, only a small (~1 cm-1) isotope shift is predicted for this band for the gas-phase complex, and while it does increase with the distortion of the BF3 subunit, we assign the peak at 836 cm-1 to the νBFs band of ClCH2CN–BF3 (i.e., not specific to either B isotopomer). We also observe a lone product band in the C-N stretching region at 2354 cm-1, and on the basis of intensity ratios with we assign it to the C-N stretching mode of ClCH2CN–BF3.


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a)

b)

c)

Figure 4: Representative nitrogen matrix spectra of the: (a) νCN, (b) a s s νBF , and (c) νBF and δBF regions containing ClCH2CN (i), BF3 (ii), and ClCH2CN with BF3 (iii) in a 2:1:1200 concentration ratio. Bands assigned to the 1:1 ClCH2CN–BF3 complex are noted with asterisks.


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Figure 5 shows representative spectra of ClCH2CN and/or BF3 in solid argon for the: (a) νCN, (b) νBFa, and (c) νBFs and δBFs regions. Due to broadening in the νBFa band, we began by observing product bands at 566, 583, 834 and 839 cm-1 in the νBFs and δBFs regions. The 566 and 583 cm-1 peaks exhibit a 4:1 intensity ratio, and an isotopic splitting of 17 cm-1 that is the comparable to that observed for free BF3 (28 cm-1) and the calculated (B3PW91/aug-cc-pVTZ) shift (20 cm-1). Thus, we assign the 566 and 583 peaks to the 11B/10B isotopic pair of the δBFs of ClCH2CN–BF3, respectively. The doublet at 834 and 839 cm-1 was presumed to be the result of site splittings, and we refer to it by its approximate center at 837 cm-1, and on the basis of intensity ratios, we assign it to the νBFs band of ClCH2CN–BF3. We, once again, also observe a single product band in the CN stretching region at 2352 cm-1 , and on the basis of intensity ratios, we assign it to the νCN mode of ClCH2CN–BF3.

In the νBFa region, we observe only a broad band at about 1300 cm-1. The absence of the isotopic counterpart makes a definitive assignment difficult, but we presumed that this feature is the 11B component of the νBFa band, because its intensity ratio is consistent with other bands, though the peak area difficult to measure with great precision. The line shape of this peak is an interesting issue in its own right, and we believe it is possible that this stems from a dynamical averaging of the structure due to large amplitude of vibrational motion along the B-N bond coordinate. We note below that the B-N bond potential is remarkably flat over a wide range of bond lengths in a dielectric continuum that is comparable to solid argon. Also, we also note that for CH3CN–BF3, in which the gas-phase B-N potential is comparably flat, that the vibrational amplitude in the B-N


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stretching mode is approximately 0.9 Å.13 Given the known sensitivity of the νBFa mode to the B-N distance,9 it is possible that the large amplitude motion may manifest the observed line broadening.


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a)

b)

c) a

Figure 5: Representative argon matrix spectra of the: (a) νCN, (b) νBF , s s and (c) νBF and δBF regions containing ClCH2CN (i), BF3 (ii), and ClCH2CN with BF3 (iii) in a 1:1:600 concentration ratio. Bands assigned to the 1:1 ClCH2CN–BF3 complex are noted with asterisks.


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Figure 6 shows representative spectra of ClCH2CN and/or BF3 in solid neon in the: νCN (a), νBFa (b), and νBFs /δBFs (c) regions. In this instance, we made our initial assignments in the νBFs /δBFs region, where we observed product bands at 591 and 611 cm-1, a with a 4:1 intensity ratio and 20 cm-1 splitting that is consistent with previously stated benchmarks. Therefore, we assign the 591 cm-1 and 611 cm-1 bands to the BF3 symmetric deformation (δBFs ) modes of ClCH2CN–11BF3 and ClCH2CN–10BF3, respectively. Also, see observed an additional product band at 861 cm-1, in the BF3 symmetric stretching region on the basis of intensity ratios, we assign this 861 cm-1 peak to the νBFs mode of ClCH2CN–BF3. In the νBFa region of these spectra, we observes only one product band at 1408 cm-1, which did exhibit intensity ratios consistent with the other assignments, so we assign it to the 11B νBFa mode. We note that the 10B isotopic counterpart of this band, expected near 1460 cm-1, would be obscured by an intense BF3 peak. Finally, we report that no product bands we observed in the C-N stretching region in these experiments.


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a)

b)

c) a

Figure 6: Representative neon matrix spectra of the: (a) νCN, (b) νBF , s s and (c) νBF and δBF regions containing ClCH2CN (i), BF3 (ii), and ClCH2CN with BF3 (iii) in a 4:1:1400 concentration ratio. Bands assigned to the 1:1 ClCH2CN–BF3 complex are noted with asterisks.


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A complete listing of frequency assignments is provided in Table 1, and as a whole, a systematic series of shifts is apparent when the data for each complex are examined across the various media, matrix, gas-phase, and solid state. Figures 7 and 8 provide a visual representation of shifts in the BF-asymmetric stretching band, the most structurally sensitive mode,9 for FCH2CN–11BF3 and ClCH2CN–11BF3, respectively. The trend is clear, in that the peaks shift systematically to lower frequencies as the ability of the medium to stabilize charge increases (i.e., solid > N2 ≥ Ar > Ne >gas). This implies that the B-N distance contracts systematically in accord with these frequency shifts, in proceeding from the gas-phase (~2.4 Å),11 through the various media, and ultimately to the solid state (~1.6 Å).11 The neon data are only slightly displaced from the gas-phase frequencies, indicating at most a rather subtle effect on the structures, but the nitrogen and argon data are displaced about 125 to 175 cm-1 from the calculated gas-phase bands. We emphasize here that for one, these shifts are systematic and occur across a range of very low-dielectric media (ε(Ne)~1.1-1.2) < ε(Ar) ≤ ε(N2)~1.4-1.5).29-32 Furthermore, the dynamic range of the frequency shifts is much larger for these complexes (∆ν (gas-solid) = ~ 225 – 250 cm-1) than for CH3CN–BF3, (∆ν (gas-solid) = ~ 110 cm-1), 9,15 which parallels the prediction of larger gas-solid structure differences for the halo-acetonitrile complexes.11 However, the matrix frequencies are still interspersed between these extremes, which indicates a broader range of structural change in the bulk media as well.


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-1

Table 1: Frequencies (in cm ) of FCH2CN–BF3 and ClCH2CN–BF3 in various media.

a

-1

Observed as a doublet at 598 and 603 cm presumably due to separate trapping b -1 c sites. Observed as a doublet at 596 and 598 cm . Observed as a quartet at 593, -1 d -1 e 599, 603, and 608 cm . Observed as a doublet at 836 and 839 cm . Observed as a -1 doublet at 1408 and 1413 cm .


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a

Figure 7: Illustration of the frequency shift of the B-F asymmetric stretch (ν across various media.

BF)

a

Figure 8: Illustration of the frequency shift of the B-F asymmetric stretch (ν across various media.


11

in FCH2CN– BF3

BF)

11

in ClCH2CN– BF3

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B-N Bond Potentials Further evidence for effects of the bulk condensed-phase media on the structural properties FCH2CN–BF3 and ClCH2CN–BF3, comes from an examination of calculated B-N bond potentials, and moreover, these data provide mechanistic insight as well. Figure 9 shows B-N potential curves for FCH2CN–BF3, as computed via several DFT methods and MP2, while those from higher-level post-HF energies (based on MP2optimized structures) are displayed in Figure 10. Figures 11 and 12 contain analogous curves for ClCH2CN–BF3. The zero-points of these potentials are set to the energy of separated, gas-phase XCH2CN (X= F, Cl) and BF3, and as such, the minimum energy point on each curve corresponds to the complex binding energy. For one, it is clear that these data are sensitive to the choice of computational method. Among the DFT methods, M06 compares most favorably to the post-HF methods in terms of overall shape relative to the minima, thus we used this method to explore the effects of bulk dielectric media on the potentials (see below).

In general, all curves exhibit a shallow minimum near 2.4 Å, which, according to the CCSD curve, reflects a binding energy of about -5.5 kcal/mol for both complexes. The key feature, however, in terms of potential medium effects on structure, is that both curves exhibit a flattened, shelf-like region between the inner wall and the minimum, the energies rising by only about 2-3 kcal/mol between the gas-phase minima and the 1.7 Å point. A similar feature is noted for HCN–BF3.6,7 By contrast, for CH3CN–BF3,13 a shelflike region extends toward longer bond distances from a minimum near 1.8 Å, but the energy range through central portion of the curve is even less, about 0.5 kcal/mol.13


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Figure 9: The B-N distance potential of FCH2CN-BF3 via various density functional methods and the aug-cc-pVTZ basis set. The energy of the separated FCH2CN and BF3 is set to 0.0 kcal/mol.

Figure 10: B-N distance potential of FCH2CN-BF3 with various post-Hartree-Fock methods using the 6311++G(2df,2pd) basis set. The energy of the separated FCH2CN and BF3 is set to 0.0 kcal/mol. MP2/aug-cc-pVTZ is included to illustrate the effect of the smaller basis set used with post-HF methods.


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Figure 11: The B-N distance potential of ClCH2CN-BF3 via various density functional methods and the aug-cc-pVTZ basis set. The energy of the separated ClCH2CN and BF3 is set to 0.0 kcal/mol.

Figure 12: B-N distance potential of ClCH2CN-BF3 with various post-Hartree-Fock methods using the 6311++G(2df,2pd) basis set. The energy of the separated ClCH2CN and BF3 is set to 0.0 kcal/mol. MP2/aug-cc-pVTZ is included to illustrate the effect of the smaller basis set used with post-HF methods.


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To model the effects of bulk, condensed-phase media on the structures of these complexes, we composed hybrid potential curves by adding the electrostatic component of the solvation free energy from PCM calculations (i.e., PCM/M06/aug-cc/pVTZ//MP2/6-311G++(2df, 2dp)) to the gas-phase electronic energy (CCSD/6311G++(2df,2dp)//MP2/6-311G++(2df,2dp)) at each point on the curve.13 Figures 13 and 14 contain these curves for both FCH2CN–BF3 and ClCH2CN– BF3, respectively. Analogous curves based solely on M06 energies for both the gas- and condensed-phases (i.e., PCM/M06, in which the structure parameters aside from B-N distance were allowed to optimize in the medium) are available as supplementary material (Figures S1 and S2), and show the same overall effects. The solvation free energy is dependent upon the dielectric constant (ε) of the medium, and the ε-values for these curves range from those typical of solid neon (ε=1.2)29 to acetone (ε=20.).25,26,31 What the curves illustrate is that the inner regions of the curves are stabilized to a greater extent by the medium as the dielectric constant increases. The solvation energy is greater at shorter bond lengths due to two major factors: more charge transfer at shorter distances, and a greater pyramidal distortion of the BF3 subunit. Both of these increase the dipole moment of the complexes (6.82 D at 1.9 Å compared to 4.99 D at 2.3 Å for ClCH2CN–BF3, via M06), and thus amplify its interaction with the medium. However, this effect is only significant in systems for which the gas-phase potential is fairly flat, such that a relatively small difference in solvation energy (~1.5 kcal/mol at ε=1.2 for FCH2CN–BF3) can alter the energy difference between the bonded (1.6-1.7 Å) and nonbonded (~2.3-2.4 Å) regions of the curve.


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Figure 13: The B-N distance potential of FCH2CN–BF3 in various dielectric media, from gas phase to ε=20. The upper curve is the gas-phase potential obtained at the CCSD/6-311++G(2df,2pd)//MP2/6311++G(2df,2pd) level. The others were obtained by adding the electrostatic component of the solvation free energy (PCM/M06/aug-cc-pVTZ//MP2/6-311++G(2df,2pd)) to the gas-phase potential. The energy of the separated, gas-phase FCH2CN and BF3 was set to 0.0 kcal/mol.

Figure 14: The B-N distance potential of ClCH2CN–BF3 in various dielectric media, from gas phase to ε=20. The upper curve is the gas-phase potential obtained at the CCSD/6-311++G(2df,2pd)//MP2/6311++G(2df,2pd) level. The others were obtained by adding the electrostatic component of the solvation free energy (PCM/M06/aug-cc-pVTZ//MP2/6-311++G(2df,2pd)) to the gas-phase potential. The energy of the separated, gas-phase ClCH2CN and BF3 was set to 0.0 kcal/mol.


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Above all else, this analysis of the potential curves provides additional support for our rationalization of the observed medium-induced frequency shifts noted above, and, together with frequencies calculated (B3PW91/aug-cc-pVTZ) for the structures at each point along the curves, we can offer some indirect, approximate insight as to the extent of the structural changes that take place in the inert media. Specifically, we note that for neon (ε=1.2),29 the frequency shifts are slight relative to the gas-phase data and the potential minima are not shifted by the medium. However, the inner wall is softened by the medium such that the vibrationally-averaged B-N distance would be a bit shorter in the dielectric medium. For both complexes, we note that the observed frequencies of the 11

B, νBFa bands compare reasonably those calculated for structures with the B-N bonds

constrained to 2.3 Å (1410-1415 cm-1). This is consistent with a slight, 0.05 to 0.1 Å contraction of the B-N bond in sold neon, relative to the gas-phase, but again this assessment is indirect, and approximate.

For an ε value of 1.5, about the value in solid argon or nitrogen,30-32 there are actually two minima on the B-N potentials of both complexes, a global minimum near that for the gas-phase, and a secondary minimum near 1.7 Å. Presuming that the barrier between these is near or below the zero-point energy in the B-N stretching coordinate, we would expect large amplitude of motion in the B-N stretch that would render an average bond distance that is significantly shorter than the gas-phase minimum. Again, calculated frequency data are reasonably consistent with a correspondingly significant structural change. The calculated νBFa frequencies for both complexes as follows: 1320-1325 cm-1 with the B-N distance constrained to 1.9 Å, 1285-1290 cm-1 at 1.8 Å, and 1250-1255 at


30

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1.7 Å. As such, the measured νBFa frequencies in argon and nitrogen are consistent with B-N bond contractions of about 0.5-0.6 Å, and 0.6-07 Å, respectively. Thus, the frequency data seem to imply that the vibrationally-averaged B-N distances would lie near the secondary minima on the ε=1.5 potential curves, which would be unlikely according to the predicted energies. Nonetheless, there is a general agreement between these assessments that a substantial contraction of the B-N bond does occur in these rather inert, bulk media. Furthermore, we note that the large-amplitude vibrational motion that would ensue along this coordinate in a medium with an ε-value near 1.5 may manifest the observed broadening of the νBFa band in the argon spectra for ClCH2CN– BF3. As a whole, however, the overall effect revealed by these curves is indeed similar to CH3CN–BF3,13 but in these cases the effects are even more extreme. This is a chemical effect due to the fact that the haloacetonitriles are weaker bases, with longer, weaker donor-acceptor bonds in the gas-phase, which allow for a larger dynamic range for the BN bond to contract.

Conclusions We have measured several IR bands for FCH2CN–BF3 and ClCH2CN–BF3 in nitrogen, argon, and neon. A comparison of frequencies in the νCN, νBFa, νBFs, and δBFs bands reveal a series of systematic shifts. In turn, these shifts indicate that the inert matrix environments cause a progressive contraction of the B-N bond occurs, in a manner that parallels the ability of the medium to stabilize the increase in polarity that ensues as the B-N contracts. We have also mapped the B-N distance potentials for FCH2CN–BF3 and ClCH2CN–BF3 using several density functional and post-Hartree Fock methods, all of


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which reveal a flat, shelf-like region that extends from the minimum (near 2.4 Å) toward the inner wall (about 1.7 Å). We were also able to rationalize the medium effects on the structure by constructing hybrid bond potentials comprised of the electrostatic component of the solvation free energy and the gas-phase electronic energy. These curves indicate that the solvation energies are greatest at short B-N distances (at which the complex is more polar), and ultimately, the potential minima shift inwards as the dielectric constant of the medium increases. This is quite consistent with our rationalization of the measured frequency shifts, and thus provides mechanistic insight into the process by which the structures of these complexes change in bulk, condensed-phase media.

Acknowledgements The work was supported by the National Science Foundation (CHE-0718164 and CHE1152820). Additional support was obtained from the Petroleum Research Fund, administered by the American Chemical Society, as well as UWEC’s Office of Research and Sponsored Programs. J.A.P. also acknowledges a Henry Dreyfus Teacher-Scholar Award from the Camille and Henry Dreyfus Foundation.

Supporting Information Available

Figures S1 and S2, which display the B-N potential of each complex in the gas-phase and dielectric media via the M06 method are available free of charge via the Internet at http://pubs.acs.org.


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References 1. See for example: Huheey, J.E. Inorganic Chemistry: Principles of Structure and Reactivity, 3rd ed.; Harper & Row: New York, NY, 1983. 2. Leopold, K.R., Canagaratna, M.; Phillips, J.A. Partially Bonded Molecules from the Solid State to the Stratosphere. Acc. Chem. Res. 1997, 30, 57-46. 3.  Leopold, K.R. Partially Bonded Molecules and Their Transition to the Crystalline State, in Advances in Molecular Structure Research; Hargittai, M., Hargittai, I., Eds.; JAI Press: Greenwich, CT, 1996; Vol. 2. 4. Haaland, A. Covalent versus Dative Bonds to Main Group Metals, a Useful Distinction. Angew. Chem. Int. Ed. Engl. 1989, 28, 992-1007. 5. Dvorak, M.A.; Ford, R.S.; Suenram, R. D.; Lovas, F.J.; Leopold,  K.R. van der Waals vs Covalent Bonding: Microwave Characterization of a Structurally Intermediate Case. J. Am. Chem. Soc. 1992, 114, 108-115. 6. Reeve, S.W.; Burns, W.A.; Lovas, F.J.; Suenram, R.D.; Leopold, K.R. Microwave Spectra and Structure of HCN–BF3: an Almost Weakly Bound Complex. J. Phys. Chem. 1993, 97, 10630-106307, 7. Burns. W. A.; Leopold, K.R. Unusually Large Gas-Solid Structure Differences in HCN– BF3. J. Am. Chem. Soc. 1993, 115, 11622-11623. 8. Wells, N. P.; Phillips, J.A. Infrared Spectrum of CH3CN–BF3 in Solid Argon. J. Phys. Chem. A 2002, 106, 1518-1523. 9. Giesen, D. J.; Phillips, J.A. Structure, Bonding, and Vibrational Frequencies of CH3CN– BF3: New Insight into Medium Effects, and the Discrepancy between the Experimental


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and Theoretical Geometries. J. Phys Chem. A 2003, 107, 4009-4018. 10. Phillips, J.A.; Giesen, D.J.; Wells, N.P.; Halfen, J.A.; Knutson, C.C.; Wrass, J.P. Condensed Phase Effects on the Structure and Bonding of C6H5CN-BF3 and (CH3)3CCNBF3: IR Spectra, Crystallography, and Computations. J. Phys. Chem. A 2005, 109, 81998208. 11. Phillips, J.A.; Halfen, J.A.; Wrass, J.P.; Knutson C.K.; Cramer, C.J. Large Gas - Solid Structural Differences in Complexes of Halo-acetonitriles with Boron Trifluoride. Inorg. Chem. 2006, 45, 722-731. 12. Eigner, A.A.; Rohde, J.A.; Knutson, C.C.; Phillips, J.A. Infrared Spectrum of CH3CN– BF3 in Solid Neon: Matrix Effect on the Structure of a Lewis Acid-Base Complex. J. Phys. Chem. B. 2007, 111, 1402-1407. 13. Cramer, C.J.; Phillips, J.A. The B-N Distance Potential of CH3CN–BF3 Revisited: Resolving the Experiment-Theory Structure Discrepancy and Modeling the Effects of Low-Dielectric Environments. J. Phys. Chem. B 2007, 111, 1408-1415. 14. Swanson, B.; Shriver, D.F. Ibers, J.A. Nature of the Donor - Acceptor Bond in Acetonitrile-Boron Trihalides. The Structures of the Boron Trifluoride and Boron Trichloride Complexes of Acetonitrile. Inorg. Chem. 1969, 8, 2182-2189. 15. Swanson, B.; Shriver, D.F. Vibrational spectra, Vibrational analysis, and bonding in Acetonitrile - Boron Trifluoride. Inorg. Chem. 1970, 9, 1406-1416. 16. Jurgens, R.; Almlöf, J. Two Very Different B…N Bond Distances: Electronic Structure Calculations on BF3…NCCN and BF3…NCCH3. Chem. Phys. Lett. 1991, 176, 263-265. 17. Jiao, H. J.; Shleyer, P.v.R. Large Effects of Medium of Geometries: An ab Initio Study. J. Am. Chem. Soc. 1994, 116, 7429-7430.


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18. Jonas, V.; Frenking, G.; Reetz, M.T. Comparative Study of Lewis Acid-Base Complexes of BH3, BF3, BCl3, AlCl3 and SO2. J. Am. Chem. Soc. 1994, 116,  8741-8753. 19. Cho, H-.G.; Cheong, B-.S. Theoretical Investigation of the Structure and Vibrational Frequencies of CH3CN–BF3 and CH3CN–BCl3. J. Mol. Struct. (THEOCHEM) 2000, 496,  185-198. 20. Beattie, I.R.; Jones, P.J. The Effect of the Surrounding Medium on the Interaction of Nitriles with Boron Trifuloride. Angew. Chem. Int. Ed. Engl. 1996, 35, 1527-1529. 21. See for example: Dunkin, I.R. Matrix Isoloation Techniques: A Practical Approach, Oxford University Press: Oxford, 1998. 22. Young, N.A. Main Group Coordination Chemistry at Low Temperatures: A Review of Matrix Isolated Group 12 to Group 18 Complexes. Coord. Chem. Rev. 2013, 257, 9561010. 23. Tomasi, J. Thirty Years of Continuum Solvation Chemistry: a Review, and Prospects for the Future. Theor. Chem. Acc. 2004, 112, 184-203. 24. Smith, E.L.; Sadowsky, D.; Cramer, C.J.; Phillips, J.A. Structure, Bonding, and Energetic Properties of Nitrile - Borane Complexes: RCN–BH3. J. Phys. Chem. A. 2011, 115, 19551963. 25. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Montgomery, Jr., J.A.; Vreven, T.; Kudin, K.N.; Burant, J.C. et al. Gaussian, Inc., Wallingford CT, 2004. 26. Frisch, M.J.; Trucks, G.W.; Schlegel, H.B.; Scuseria, G.E.; Robb, M.A.; Cheeseman, J.R.; Scalmani, G.; Barone, V.; Mennucci, B.; Petersson, G.A. et al. Gaussian, Inc., Wallingford CT, 2009.


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27. For a recent overview of most of the computational method and basis sets referred to herein, see: Cramer, C.J. Essentials of Computational Chemistry, 2nd ed.; John Wiley and Sons: Chichester, U.K., 2004; see also references therein. 28. Zhao, Y.; Truhlar, D.G. The M06 Suite of Density Functionals for main group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: two new Functionals and Systematic Testing of Four M06-class Functionals and 12 other Functionals. Theor. Chem. Accts. 2008, 120, 215-241. 29. The ε value at the critical point is 1.07: Chan, M. H. Dielectric constant of Ne near its liquid-vapor critical point. Phys. Rev. B. 1980, 21, 1187-1193. The value for the solid is likely to be slightly larger based on the results in ref 32. 30. The value for liquid nitrogen is 1.45 (ref. 31). The value for solid nitrogen is likely to be slightly larger based on the results in ref 32. 31. CRC Handbook of Chemistry and Physics, 65th ed.; CRC Press: Boca Raton, FL, 1984. 32. Amey, R. L.; Cole, R.H. Dielectric Constants of Liquefied Nobel Gasses and Methane. J. Chem. Phys. 1964, 40, 146-149. 33. Phillips, J.A.; Cramer, C.J. Quantum Chemical Characterization of the Structural Properties of HCN–BF3. J. Chem. Theory Comput. 2005, 1, 827-833.


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